I'm not making myself clear. The marker lights were being powered from the trailer battery via the gash in the cord that runs to the TV (the one that was not plugged in).

A-h-h-h...now I get it!

Somehow, once the pigtail line was gashed, the marker wire came into contact with the chargeline wire, which is energized by either/both the trailer and the tow vehicle- right?
And as suggested above, water could have played a role- the perfect conductor if it got into that gash in the pigtail.

Thanks for sticking with me, folks!
Electricity 101 is now excused for the day...

Thomas said: My guess is that when the wire was gashed, some of the copper strands were overlapped between the wire carrying 12 volts (from TV to trailer battery) and the wire carrying power from the TV to the marker lights.

It Does Not have to be connected to the tow vehicle because the same wire that gets 12VDC from the TV is also connected to the camper battery and if the wires were cut and a strand from the +12VDC happened to make contact with a strand that goes to the light, it will light.